1. Field of the Invention
This invention relates to liquid crystal compositions and more particularly, to guest-host liquid crystal compositions comprising indigo dyes as a guest dye.
2. Description of the Prior Art
Liquid crystal displays making use of the guest-host interaction effect are generally called guest-host (hereinafter referred to simply as G-H) displays. This type of display makes use of a G-H liquid crystal composition in which a dichroic or pleochroic dye is dissolved in a host liquid crystal. The host liquid crystal materials may include a variety of liquid crystal materials such as nematic liquid crystals, chiral nematic liquid crystals, cholestric liquid crystals, smectic A liquid crytals and chiral smectic C liquid crystals and the like.
As is known in the art, the contrast of the G-H liquid crystal displays is largely influenced by the order parameter, S, of the dichroic dye in the host liquid crystal. This parameter can be experimentally determined according to the following equation EQU S=(A.sub..parallel. -A.sub..perp.)/A.sub..parallel. +2A.sub..perp.)
in which A.sub..parallel. and A.sub..perp. are, respectively, absorbances of the dye obtained by passing linear polarized light along the directions parallel and perpendicular to the direction of orientation (or the director direction) of the host liquid crystal. In G-H liquid crystal display devices using parallel dichroic dyes (P-type dyes), a display of a higher contrast becomes possible as the order parameter, S, comes closer to 1.
On the other hand, the characteristics required for pleochroic dyes used in G-H liquid crystal displays include, aside from the order parameter, S, a high solubility in host liquid crystal and a high durability on application as a display device.
Most pleochroic dyes ordinarily used in G-H liquid crystal displays are azo and anthraquinone dyes with a variety of chemical structures.
In spite of the fact that indigo dyes have generally good durability, they have never been known for use in G-H liquid crystal display devices as pleochroic dyes. The most serious problem involved in indigo dyes on use as a dichroic dye for G-H liquid crystal display is that they have a very low solubility in host liquid crystal. This means that the G-H liquid crystal compositions comprising indigo dyes dissolved in host liquid crystals are rarely colored and cannot thus be applied for G-H liquid crystal displays.